Lap and Lead in Slide Valves

Valves and valve gears can be the making or the breaking of the performance of any engine. There are other things that enter the picture as well, but, if the valves and valve gear are right, the other conditions can be dealt with and usually corrected without too much trouble.

It could be that an explanation and a sketch or two might be welcome. I realize that I am treading on dangerous ground here, as a complete answer to valve and valve gear problems takes a lifetime of study to master and a lot of technical study, in addition.

When talking about this subject, one will hear some of the terms mentioned, such as lap, lead, valve travel, steam ports, exhaust port, bridges and line in line. To a good many of the older (I do not mean in years) and more experienced Live-Steamers, these terms are clear and well understood. To the man building his first engine, and some of the more experienced ones as well, they are about as clear as mud and not too well understood.

Lap is the amount the steam edges of the valve overlap the steam ports when the valve is central. Its purpose is to allow the steam to be used expansively. This means less fuel and water is used.

Lead is the amount the valve has opened the steam port for admission when the piston is on a dead center. It is given for the purpose of getting the steam into the cylinder so that it will be there ready to push the piston as it passes the dead centers.

Valve Travel is the total distance that the valve moves in one direction and measured with the reverse lever in either full gear position. it should be equal to one lap plus one steam port times two. For example, if one lap is 1/8 inch and one steam port is 1/8 inch, then the total full gear travel should be one-half inch. It is important to know the total full gear valve travel so that the valve gear itself can be designed to give this full gear valve travel.

The steam ports, the exhaust port and the bridges are best explained by Sketch #1 and the Port Face. The bridges should be equal to the steam ports. The exhaust port should be at least twice the size of the steam ports, measured fore and aft.

Line and Line is a somewhat ambiguous term, as it is used in two different ways. Sometimes it is applied to a valve that has no lap. In this case, the steam edge of the valve and the steam port are line and line. Such a valve would permit steam to follow the piston its whole stroke and would be extremely wasteful.

It is my belief that this term is more commonly used in connection with the exhaust edges of the valve and the exhaust edge of the steam ports. This is the condition shown on Sketch #1. The valve cavity should always be this way, if it is a trifle more, it will help the steam to get out easily, and this is just as important as getting it in.

Lap and Lead in Piston Valves

Below is a sketch showing the way the valves are setup on the 1-1/2 inch scale Berkshire. This is a typical setup for modern steam locomotives which were equipped with inside admission piston valves. I didn't work any magic here. I just scaled down the work done by the steam design engineers of the era who figured out the optimum performance characteristics of these superpower locomotives.

On the model:

lap = 0.211 inch

lead = 0.030 inch

exhaust clearance = 0.008 inch

The piston valves on the model are 1-1/2 inch diameter and the valve gear is designed to provide 1 inch max valve travel.

At the piston front and back dead center the valve gear imparts no motion to the valve, or no offset from the valve centerline. That is done by the "combination lever", which offsets the valve from centerline an amount equal to the lap plus lead. The valve gear at these points is simply along for the ride.

Lead is the amount the steam edge port is open when the main piston is at the end or beginning of its stroke. It ensures that there is an unrestricted supply of steam to the cylinder shortly after the piston begins its stroke. It is generally larger on high speed passenger engines than on locomotives designed for freight service. (Ed: please explain why)

Steam lap is there to cutoff the cylinder steam supply to the cylinder before the piston reaches the end of its stroke and thus take advantage of the expansive force of the steam, increasing the locomotives efficiency. The delay of steam admission caused by the steam lap also contributes to ensuring that the steam is thoroughly exhausted from the other side of the piston before steam admission occurs.

The exhaust edges and rings are generally setup in two of three possible ways:

Line & line: with the valve on center, if the exhaust edges of the rings coincide with the exhaust edges of the ports, the valves are said to be "line & line". This arrangement is prevalent on slower speed freight engines.

Exhaust lead: with the valve on center, if the cylinder is open to the exhaust, the valve has exhaust lead. This serves to decrease the expansion of the steam but also contributes to the exhaust event occurring longer. Exhaust lead, if applied, is larger on passenger engines, but less in freight service.

Exhaust lap: with the valve on centerline, if the exhaust edges of the rings overlap the exhaust edges of the ports, the valve has exhaust lap. With exhaust lap, the duration of the exhaust is shortened and the duration of steam expansion is lengthened. Exhaust lap was rarely applied in prototype engines.

As stated above, modern prototype steam locomotives were equipped with inside admission piston valves as shown in the sketch. Many late steam-era live steam locomotive models, however, are equipped with outside admission slide valves. They work fine but are 180 degrees out of phase from what the prototype had and required that the eccentric crank "lead" the main crank in forward motion rather than "follow", with valve or radius rod set below the valve stem rather than above. Lap and lead still apply as for outside admission except that steam lap and lead are now applied to the outside or steam edges of the valve while exhaust clearance, lap or "line & line" are applied to the inside.

Square

What is meant by the term Square or Out of Square applying to a locomotive?

When an engine is Square it means that the valve setting is such that the four exhaust blasts, which occur with each revolution of drive wheels, are evenly spaced and of equal intensity. Being Out of Square is the reverse of this condition and indicates some trouble with the valve gear (or valve setting).

Troubleshooting

Several issues can lead to a locomotive being out of square.

Valve Timing

Tumbler Rods

Setting the lifting arms on to the tumbling shaft. This is critical before you bore a #3 taper pin reamer to finalize the position. If one side is off, the block in the link will be off position from the other side and the choo choo will walk with a limp.

Grinding

I have built several of the Allen Moguls and Ten Wheelers. When I would work up the cylinders, heads, steam chests, and covers, I would lap the heads to the cylinders with fine grade of Clover valve grinding compound. The steam chests and covers would be worked over a surface plate, e.g. a piece of broken window glass that I got out of an old railroad passenger car, along with a really fine grade of wet or dry sandpaper with a bit of kerosene. The parts would be assembled with no gaskets, just a little smear of non hardening permetex, and, of course, the necessary bolts needed to hold everything together.